The present invention relates to a toner for electrophotography used in an electrophotographic copying machine, an electrophotographic facsmile, and an electrophotographic printer, etc., and in particular, relates to a toner preferable for carrying out fixing by flash light.
Since fixing of a toner image on a recording medium is performed without applying pressure to the toner in an electrophotographic apparatus for performing image formation using flash light, there is a great need for a color toner that is well melted by efficiently utilizing energy from the flash light.
An electrophotographic method that has been conventionally and broadly adopted includes respective processes of (1) electrostatically charging of a photoconductor, (2) light exposure of the photoconductor (formation of a latent image), (3) development of the latent image by a toner, (4) transcription of the toner image to a recording medium, and (5) fixing of the toner image on the recording medium.
Among these, as a fixing method for a toner image, a heat-roll method such that the toner is heated directly between heated rollers, a oven-fixing method, and a flash-fixing method such that fixing is performed by light irradiation and far-infrared-ray irradiation are well known.
Among the fixing methods, the heat-roll method is most broadly adopted. In this heat-roll method, since a simple configuration is provided such that fixing is accomplished by heating and directly applying pressure to the toner by the rollers at high temperature, there are advantages in that the apparatus is inexpensive and a surface for fixing the toner can be made flat. On the contrary, there are many known problems in that a paper (recording medium) after fixing becomes rolled, dirt on the paper is easily generated by an offset since toner dirties the surfaces of the fixing rollers, ultra-speeding up is difficult because of the rolling of the paper, and fixing to a special recording medium such as a sealable postcard is difficult, etc.
In contrast to the heat-roll method, the flash-fixing method, which fixes the toner without any contact, is free of the problem of paper rolling or offset and also easy to adapt for speeding up and the sealable postcard, thus being used for business high-speed printers and high-speed copying machines.
In the flash-fixing method, when the toner used is black, fixing can be performed relatively easily by making the temperature on the outermost surface of a toner particle equal to or more than 200xc2x0 C. However, in the case of a color toner, since flash light is transmitted, the temperature of the outermost surface may be equal to or more than 100xc2x0 C., so that the situation occurs in which it is not certain that fixing will be carried out.
By the way, an external additive is commonly added to the surface of the toner particle for the purpose of improving fluidity an d the electrostatic property of the toner. As this external additive, it is common to use a white fine particle such as titanium oxide, silica, alumina, etc. However, in the case of flash-fixing, since these white particles of external additives further reflect flash light, the fixing property of the toner is further degraded. Furthermore, a black external additive is also needed for the black toner in view of preventing the fixing property from degrading.
Also, with regard to a color toner, the method of adding an infrared-ray absorbent is known so that fixing by flash light is carried out with certainty (for example, see Japanese Laid-Open Patent Application No. 7-191492). However, since such an infrared-ray absorbent is expensive and also it is difficult to produce an effective infrared-ray absorbent, it is desired to decrease it as much as possible.
Then, as a countermeasure to the problem for the white external additives and the decrease in the infrared-ray absorbent, a proposition of a technique for improving the fixing property by employing black external additives such as carbon black etc. has been made so as to enhance the efficiency for energy absorption, but in the case of the color toner, there occurs the problem that the color becomes turbid after fixing and thus they cannot simply be employed.
Furthermore, although a technique employing titanium black as an external additive is proposed in Japanese Laid-Open Patent Application 6-332233, in the case of the color toner, there is the problem that the color becomes turbid after fixing and there is also a problem of the toner lacking in environmental stability.
Accordingly, it is a main object of the present invention to provide a toner of which fixing can be performed efficiently and certainly by flash light, with environmental stability.
The above object is achieved by a toner having a surface to which a titanium black fine particle adheres which fine particle has a property of being changed in color from black to white by flash light, according to the present invention.
According to the invention described above, when fixing is performed by the flash light, since the titanium black fine particle of a black color adheres to the surface of the toner particle, light energy is converted into heat efficiently so that the toner is fixed on a recording medium and the titanium black fine particle changes color from black to white by the temperature rise thereat. Thus, the problem that an image after fixing is black and turbid is resolved. Moreover, in the case of the color toner, the advantage in that the amount of the infrared-ray absorbent can be decreased occurs.
It is preferable that for the toner the titanium black fine particle be subjected to a hydrophobicity-providing treatment with a titanate-based, silicone-based, or aluminum-based coupling treatment agent. According to such a toner, since the titanium black fine particle is subjected to a hydrophobicity-providing treatment, the environmental stability of the toner is improved and a good electrostatic property can be maintained over a long period.
Also, it is preferable that for the toner an average first-order particle diameter of the titanium black fine particles be 0.005 through 0.04 xcexcm. Titanium black is represented by the general formula TinO(2nxe2x88x921), and one of which the first-order particle diameter is 0.005 through 0.04 xcexcm is preferred as an external additive to a toner for flash-fixing since the color changes from black to white at a temperature equal to or less than 200xc2x0 C.
Also, it is preferable that for the toner, the color changing temperature from black to white be within a range of 70xc2x0 C. through 200xc2x0 C. Such a toner can be whitened in a broad range of energy of the flash light. Such a titanium black fine particle can be produced by adjusting a lattice defect of a titanium black crystal and the particle diameter.
Also, it is preferable for the toner to contain at least a polyester resin of which a raw material is an alkylene oxide adduct of bisphenol A represented by the following formula (1): 
(In the formula described above, R represents an ethylene or propylene group and X, Y represent integers equal to or more than 1, respectively.)
As a polyester resin including the monomer as described above is employed as a binder resin for the toner, a more preferable toner can be provided since there is a little decomposed product and little odor is generated in the fixing by the flash light.
Also, it is preferable for the toner to contain 0.01 through 10 parts by weight of a compound represented by the following formula (2):
Cxe2x80x94[CH2xe2x80x94Oxe2x80x94COxe2x80x94(CH2)nxe2x80x94CH3]4xe2x80x83xe2x80x83(2)
per 100 parts by weight of a binder resin (n is an integer equal to or more than 14.)
Since the compound described above functions as a fixing auxiliary and improves the flash-fixing property of the toner, containing a certain amount in the toner is preferred.
Also, in the toner, when this is a color toner, an infrared-ray absorbent may be further contained. For this infrared-ray absorbent, its amount can be decreased compared to the conventional case.
Then, it is recommended that the infrared-ray absorbent be one selected from the group including aminum, diimmonium, naphthalocyanine, and tin oxide. These are recommended in that the efficiency of infrared absorption is good, in particular, diimmonium is recommended.
Moreover, the present invention includes an image formation method including a process of fixing a toner image on a recording medium onto a surface thereof by flash exposure, in which energy of the flash light is 0.5 through 3.0J/cm2, its luminous time is 500 through 3000 xcexcs, and a toner having a surface to which a titanium black adheres is employed, which titanium black is subjected to a hydrophobicity-providing treatment and changed in color from black to white by the flash light.
According to the image formation method as described above, high quality image formation can be performed efficiently by utilizing the color change of titanium black.